spi: cadence_qspi: add device tree binding doc

This patch adds the device tree binding doc for the cadence qspi controller &
also removes the not needed properties from the stv0991 device tree.

Signed-off-by: Vikas Manocha <vikas.manocha@st.com>
Reviewed-by: Jagannadh Teki <jteki@openedev.com>